(SMTPREVERSEPATH a.k.a. "reverse_path") if it didn't already have one.
That's because smb_getmsghdr() will point the msg.reverse_path convenience
pointer to the from_net_addr if there was no explicit reverse-path (e.g. RFC822's
"return-path" header field). This could manifest itself in *any* change to a
message header via JS failing with an "illegal header length increase" error if
the added header field just happen to put the total header length over the
allocation threshold of the pre-existing msg header.

Fix: only model a msg header "reverse_path" property if the header field
actually existed (not based on the SMBLIB convenience pointer).
When the expand_fields option is used, the old behavior remains but expanded
headers cannot be written back to the base, so no harm there.

Reported by Coz in #synchronet from failed runs of scrubmsgs.js. Thanks!